1. Field of the Invention
The present invention relates generally to off-gas stacks of furnaces, melters and reaction vessels. More specifically, the present invention relates to methods and apparatus for cooling off-gas stacks and preventing particulate from exiting the off-gas stack of a furnace, melter or reaction vessel.
2. Discussion of Background
A furnace, melter or reaction vessel generally comprises a vessel having a lower portion with means for heating material, an upper portion or "plenum" with a device for adding feed materials, and an off-gas stack. Various gases and vapors plus heat will exit the plenum through the off-gas stack. If the feed is injected in the form of particles into the vessel or because of reactions taking place within the vessel, smaller particles of feed and reactants may be swept up into the stack before reaching the melt.
When air and gases flow from the relatively larger volume of the plenum to the smaller volume of the off-gas stack, recirculating currents are established near the inside wall of the stack near its entrance. Particles entrained in these recirculating currents tend to stick to the stack wall, to accumulate there, and to restrict gas flow. The attached particulate can obstruct flow as well as cause corrosion of the stack wall in time. The smallest particulate, however, will be swept through and out of the stack.
Also, the wall at the stack entrance will be at a relatively higher temperature than the downstream portions of the stack wall since the air and gas flow will be hotter upon leaving the plenum and cooler as it moves up the stack. Therefore, the stack entrance will see more thermal stresses than the down stream portions.
There is a need for a means for preventing particulate from attaching to the stack wall and for cooling the stack wall.